Official population projections for Hong Kong until 2039 received little attention when published late last month. Yet they should be the basis of debate about society's choices as well as a basis for public investment priorities.

They suggest that, although the birth rate remains at half the replacement level, total population will continue to grow by an average of 0.8 per cent a year and reach 8.89 million by 2039, compared with 7 million today. They assume a continuation of existing trends and policies.

There are four reasons for the projected increase. The two not directly subject to government action are:

Increased life expectancy, which is projected to rise from 79.8 years to 83.7 years for men and from 86.1 to 90.1 years for women. This may prove overly optimistic as pollution and diet issues offset improvement in medicine. But it is a reasonable guesstimate.

Percentage of those born here to mainland parents who remain in or return to Hong Kong. With these now constituting 35 per cent of all births, this is critical - but unknowable, as it will be determined by relative educational and living conditions between Hong Kong and Guangdong. It could well be less and is unlikely to be more than the 62 per cent assumed.

Two reasons can be directly influenced by government:

The numbers of one-way permit holders, now 150 a day or roughly 54,750 a year. This has been static since 1995. Both the number and composition could be changed by agreement with the mainland. This issue never gets aired, even though it is an increasingly important component of population growth and mix.

The number of foreign domestic helpers. This is assumed to keep growing faster than the population as a whole as Hong Kong gets richer and the numbers of elderly people demand more carers, reaching 389,000 compared to 265,000 today. Whether it is socially desirable is one question. Another is whether the government will use immigration rules or pay scales to limit the number.

The overall 2039 total seems likely to be lower than the projection, rather than higher, particularly if the number of 'mobile residents' - those who are actually living on the mainland, in Canada and the like - rises.

But beyond guesswork there should be debate: what does Hong Kong want, given its geography, its age structure, its economic base? Does it really agree with 'bigger is better' notions dear to Chief Executive Donald Tsang Yam-kuen with his vision of a city of 10 million? That is the way Singapore had been going until recent outcries. Singapore's population has risen dramatically from 3 million in 1990 to 4.99 million today. Much of the increase has been in the number of foreign workers, mostly unskilled, with no residence rights.

Hong Kong might do better to raise its own birth rate. The projection suggests a further fall from the already very low fertility rate of 1.042 births per woman. A revival seems improbable while government land and social policies make the opportunity cost of having children so high. But investment in births would make for a more youthful community than relying on the return of babies born to mainland parents. Even with the rather optimistic estimate of a 62 per cent return rate, and a continuing rise in the number of domestic helpers, mostly in the 25-40 age group, the median age will rise by seven years, to 47.6 by 2039.

Of course, Hong Kong's increasing integration with the mainland will affect its demographics. But given the rapid ageing on the mainland, Hong Kong needs to look more to its own human resource potential - and to welcoming non-ethnic Chinese by granting permanent residency to business and professional people from India, the West and elsewhere.

Let's have a real debate about responding to the implications of this document.